This invention concerns a dry metallized film capacitor which includes an interrupter and is potted with a plasticized urethane containing unreacted isocyanate groups. Sufficient plasticizer is used so that the potting material is soft enough to permit functioning of the interrupter and is also viscous enough to remain in place to prevent spillage under leakage or rupture conditions.
The use of an interrupter in a metallized film capacitor is well-known. Its function is to force the electrode tabs to be disconnected from the terminals in the case of rapid pressure build-up within the capacitor. When such pressure builds up, the capacitor cover bearing the terminals bulges; since the ends of the terminals are force-fit through holes in the interrupter, the upward movement of the cover carries the interrupter with it, breaking the connection of the tabs from the bottoms of the respective terminals.
Potting materials have been used in wrapped capacitors, in encapsulated units, and as fill in plastic-cased units. In these applications, the potting material serves to protect the unit from end-fill material and fill irregular spaces or provide vibration protection. Heretofore, potting compounds have not been used in dry metallized film capacitors requiring a fail-safe interrupter as they have not been soft enough to prevent uneven pressure buildup, e.g., by trapping a pocket of gas, and, as a result, interfered with the proper operation of the interrupter.